307 research outputs found
Giant Planet Formation, Evolution, and Internal Structure
The large number of detected giant exoplanets offers the opportunity to
improve our understanding of the formation mechanism, evolution, and interior
structure of gas giant planets. The two main models for giant planet formation
are core accretion and disk instability. There are substantial differences
between these formation models, including formation timescale, favorable
formation location, ideal disk properties for planetary formation, early
evolution, planetary composition, etc. First, we summarize the two models
including their substantial differences, advantages, and disadvantages, and
suggest how theoretical models should be connected to available (and future)
data. We next summarize current knowledge of the internal structures of solar-
and extrasolar- giant planets. Finally, we suggest the next steps to be taken
in giant planet exploration.Comment: Accepted for publication as a chapter in Protostars and Planets VI,
to be published in 2014 by University of Arizona Pres
Problems and perspectives of formation of agricultural clusters for increasing food security of developing countries
The authors of the article develop the proprietary methodology of integral approach to formation of the projects and programs of socio-economic development, which allows differentiating the sub-system of previously viewed indicators as to the level of users’ demands. The sense of cluster approach to increasing the competitiveness of agro-industrial complex is analyzed, and the problems and perspectives for formation of agricultural clusters for the growth of food security of developing countries are determined. As a result of the research, the authors came to the conclusion that agricultural clusters are a perspective course of increasing food security of developing countries, as they improve competitiveness of domestic agricultural producers and increase food independence and food self-sufficiency of economic systems. However, implementation of internal cluster cooperation requires reconsidering the existing notions of management, related to the level of methods and control, protection of intellectual property, education, and integration. The created system of relations should allow economic agents to keep balance between self-organizing and manageable behavior, combining innovational diversity and freedom of actions with the necessary level of integration. Intercompany cooperation in this regard should be similar to cooperation of ecosystem elements.peer-reviewe
Modelling supermassive black hole growth: towards an improved sub-grid prescription
Accretion onto supermassive black holes (SMBHs) in galaxy formation
simulations is frequently modelled by the Bondi-Hoyle formalism. Here we
examine the validity of this approach analytically and numerically. We argue
that the character of the flow where one evaluates the gas properties is
unlikely to satisfy the simple Bondi-Hoyle model. Only in the specific case of
hot virialised gas with zero angular momentum and negligible radiative cooling
is the Bondi-Hoyle solution relevant. In the opposite extreme, where the gas is
in a state of free-fall at the evaluation radius due to efficient cooling and
the dominant gravity of the surrounding halo, the Bondi-Hoyle formalism can be
erroneous by orders of magnitude in either direction. This may impose
artificial trends with halo mass in cosmological simulations by being wrong by
different factors for different halo masses. We propose an expression for the
sub-grid accretion rate which interpolates between the free-fall regime and the
Bondi-Hoyle regime, therefore taking account of the contribution of the halo to
the gas dynamics.Comment: 7 pages, 3 figures. Accepted for publication in MNRA
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Diversity and Paleodemography of the Addax (Addax nasomaculatus), a Saharan Antelope on the Verge of Extinction.
Since the 19th century, the addax (Addax nasomaculatus) has lost approximately 99% of its former range. Along with its close relatives, the blue antelope (Hippotragus leucophaeus) and the scimitar-horned oryx (Oryx dammah), the addax may be the third large African mammal species to go extinct in the wild in recent times. Despite this, the evolutionary history of this critically endangered species remains virtually unknown. To gain insight into the population history of the addax, we used hybridization capture to generate ten complete mitochondrial genomes from historical samples and assembled a nuclear genome. We found that both mitochondrial and nuclear diversity are low compared to other African bovids. Analysis of mitochondrial genomes revealed a most recent common ancestor ~32 kya (95% CI 11-58 kya) and weak phylogeographic structure, indicating that the addax likely existed as a highly mobile, panmictic population across its Sahelo-Saharan range in the past. PSMC analysis revealed a continuous decline in effective population size since ~2 Ma, with short intermediate increases at ~500 and ~44 kya. Our results suggest that the addax went through a major bottleneck in the Late Pleistocene, remaining at low population size prior to the human disturbances of the last few centuries
Diversity and Paleodemography of the Addax (<i>Addax nasomaculatus</i>), a Saharan Antelope on the Verge of Extinction.
Since the 19th century, the addax (Addax nasomaculatus) has lost approximately 99% of its former range. Along with its close relatives, the blue antelope (Hippotragus leucophaeus) and the scimitar-horned oryx (Oryx dammah), the addax may be the third large African mammal species to go extinct in the wild in recent times. Despite this, the evolutionary history of this critically endangered species remains virtually unknown. To gain insight into the population history of the addax, we used hybridization capture to generate ten complete mitochondrial genomes from historical samples and assembled a nuclear genome. We found that both mitochondrial and nuclear diversity are low compared to other African bovids. Analysis of mitochondrial genomes revealed a most recent common ancestor ~32 kya (95% CI 11-58 kya) and weak phylogeographic structure, indicating that the addax likely existed as a highly mobile, panmictic population across its Sahelo-Saharan range in the past. PSMC analysis revealed a continuous decline in effective population size since ~2 Ma, with short intermediate increases at ~500 and ~44 kya. Our results suggest that the addax went through a major bottleneck in the Late Pleistocene, remaining at low population size prior to the human disturbances of the last few centuries
An alternative origin for debris rings of planetesimals
Core Accretion, the most widely accepted scenario for planet formation,
postulates existence of km-sized solid bodies, called planetesimals, arranged
in a razor-thin disc in the earliest phases of planet formation. In the Tidal
Downsizing hypothesis, an alternative scenario for formation of planets, grain
growth, sedimentation and formation of planetary cores occur inside dense and
massive gas clumps formed in the outer cold disc by gravitational instability.
As a clump migrates inward, tidal forces of the star remove all or most of the
gas from the clump, downsizing it to a planetary mass body. Here we argue that
such a clump may form not only the planetary core but also numerous smaller
bodies. As an example, we consider the simplest case of bodies on circular
orbits around the planetary core in the centre of the gas clump. Bodies smaller
than 1 km suffer a strong enough aerodynamic drag, spiral in and accrete onto
the solid core rapidly; bodies in the planetesimal size range lose their
centrifugal support very slowly. We find that planetesimals orbiting the
protoplanetary core closely remain gravitationally bound to it; these may be
relevant to formation of satellites of giant planets. Planetesimals on more
distant orbits within the host clump are unbound from the protoplanet and are
set on mildly eccentric heliocentric orbits, generically forming wide rings.
These may correspond to debris discs around main sequence stars and the Kuiper
belt in the Solar System. For the latter in particular, our hypothesis
naturally explains the observed sharp outer edge and the "mass deficit" of the
Kuiper belt.Comment: Submitted to MNRA
Are SMBHs shrouded by "super-Oort" clouds of comets and asteroids?
The last decade has seen a dramatic confirmation that an in situ star
formation is possible inside the inner parsec of the Milky Way. Here we suggest
that giant planets, solid terrestrial-like planets, comets and asteroids may
also form in these environments, and that this may have observational
implications for Active Galactic Nuclei (AGN). Like in debris discs around main
sequence stars, collisions of large solid objects should initiate strong
fragmentation cascades. The smallest particles in such a cascade - the
microscopic dust - may provide a significant opacity. We put a number of
observational and physical constraints on AGN obscuring torii resulting from
such fragmentation cascades. We find that torii fed by fragmenting asteroids
disappear at both low and high AGN luminosities. At high luminosities, , where is the Eddington limit, the AGN radiation
pressure blows out the microscopic dust too rapidly. At low luminosities, on
the other hand, the AGN discs may avoid gravitational fragmentation into stars
and solids. We also note that these fragmentation cascades may be responsible
for astrophysically "large" dust particles of approximately micrometer sizes
that were postulated by some authors to explain unusual absorption properties
of the AGN torii.Comment: a typo in the title correcte
The Long-Baseline Neutrino Experiment: Exploring Fundamental Symmetries of the Universe
The preponderance of matter over antimatter in the early Universe, the
dynamics of the supernova bursts that produced the heavy elements necessary for
life and whether protons eventually decay --- these mysteries at the forefront
of particle physics and astrophysics are key to understanding the early
evolution of our Universe, its current state and its eventual fate. The
Long-Baseline Neutrino Experiment (LBNE) represents an extensively developed
plan for a world-class experiment dedicated to addressing these questions. LBNE
is conceived around three central components: (1) a new, high-intensity
neutrino source generated from a megawatt-class proton accelerator at Fermi
National Accelerator Laboratory, (2) a near neutrino detector just downstream
of the source, and (3) a massive liquid argon time-projection chamber deployed
as a far detector deep underground at the Sanford Underground Research
Facility. This facility, located at the site of the former Homestake Mine in
Lead, South Dakota, is approximately 1,300 km from the neutrino source at
Fermilab -- a distance (baseline) that delivers optimal sensitivity to neutrino
charge-parity symmetry violation and mass ordering effects. This ambitious yet
cost-effective design incorporates scalability and flexibility and can
accommodate a variety of upgrades and contributions. With its exceptional
combination of experimental configuration, technical capabilities, and
potential for transformative discoveries, LBNE promises to be a vital facility
for the field of particle physics worldwide, providing physicists from around
the globe with opportunities to collaborate in a twenty to thirty year program
of exciting science. In this document we provide a comprehensive overview of
LBNE's scientific objectives, its place in the landscape of neutrino physics
worldwide, the technologies it will incorporate and the capabilities it will
possess.Comment: Major update of previous version. This is the reference document for
LBNE science program and current status. Chapters 1, 3, and 9 provide a
comprehensive overview of LBNE's scientific objectives, its place in the
landscape of neutrino physics worldwide, the technologies it will incorporate
and the capabilities it will possess. 288 pages, 116 figure
Progressive skin fibrosis is associated with a decline in lung function and worse survival in patients with diffuse cutaneous systemic sclerosis in the European Scleroderma Trials and Research (EUSTAR) cohort.
Objectives To determine whether progressive skin fibrosis is associated with visceral organ progression and mortality during follow-up in patients with diffuse cutaneous systemic sclerosis (dcSSc). Methods We evaluated patients from the European Scleroderma Trials and Research database with dcSSc, baseline modified Rodnan skin score (mRSS) ≥7, valid mRSS at 12±3 months after baseline and ≥1 annual follow-up visit. Progressive skin fibrosis was defined as an increase in mRSS >5 and ≥25% from baseline to 12±3 months. Outcomes were pulmonary, cardiovascular and renal progression, and all-cause death. Associations between skin progression and outcomes were evaluated by Kaplan-Meier survival analysis and multivariable Cox regression. Results Of 1021 included patients, 78 (7.6%) had progressive skin fibrosis (skin progressors). Median follow-up was 3.4 years. Survival analyses indicated that skin progressors had a significantly higher probability of FVC decline ≥10% (53.6% vs 34.4%; p<0.001) and all-cause death (15.4% vs 7.3%; p=0.003) than non-progressors. These significant associations were also found in subgroup analyses of patients with either low baseline mRSS (≤22/51) or short disease duration (≤15 months). In multivariable analyses, skin progression within 1 year was independently associated with FVC decline ≥10% (HR 1.79, 95% CI 1.20 to 2.65) and all-cause death (HR 2.58, 95% CI 1.31 to 5.09). Conclusions Progressive skin fibrosis within 1 year is associated with decline in lung function and worse survival in dcSSc during follow-up. These results confirm mRSS as a surrogate marker in dcSSc, which will be helpful for cohort enrichment in future trials and risk stratification in clinical practice
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